Apical Dominance
நுனி ஆதிக்கம்
Stop assuming plants grow randomly. The apical bud is a dictator; it suppresses lateral growth to ensure vertical dominance. If you don't understand the hormonal hierarchy here, you will lose marks on auxin-related questions.
The idea
Apical dominance is the phenomenon where the apical bud inhibits the growth of axillary buds. This is primarily mediated by the hormone Auxin, synthesized at the shoot tip. Decapitation, or the removal of the shoot tip, eliminates the source of Auxin. Consequently, lateral buds are released from inhibition and begin to develop.
Why it matters
It exists to allow the plant to focus energy on vertical growth to reach sunlight before branching out, ensuring survival in competitive environments.
NEET lens
NEET focuses on the role of Auxin in this process. Remember: Decapitation equals lateral branching. Expect questions linking this to tea plantation or hedge-making practices. Do not confuse this with Gibberellins; focus on the inhibitory nature of Auxin on axillary buds.
Clinical significance
In biological systems, this is analogous to contact inhibition in human cells. Just as the apical bud suppresses lateral growth to maintain structural integrity, healthy cells stop dividing upon physical contact to prevent tumor formation. Failure of such regulatory mechanisms leads to uncontrolled proliferation, the hallmark of malignancy.
Introduction
Apical dominance is the phenomenon where the growing shoot apex (apical bud) inhibits the growth of lateral (axillary) buds, maintaining the plant's main axis and limiting branching.
Core concept
Mechanism: IAA (auxin) produced at shoot apex moves basipetally. High IAA concentration at lateral buds inhibits their growth (roots are highly auxin-sensitive → inhibited by concentrations that stimulate shoot growth). Cytokinin antagonizes this: cytokinins promote lateral bud growth. Ratio of IAA:CK determines branching. Removing apex → IAA source removed → cytokinin promotes lateral bud growth → branching. ABA also contributes to lateral bud dormancy.
Go deeper
Auxin:Cytokinin ratio model (Skoog and Miller, 1957): high IAA:CK → root formation or suppression of lateral buds. Low IAA:CK → shoot formation, lateral bud growth. This ratio is the key control mechanism. Decapitation experiments in tissue culture confirmed this ratio-dependent response.
From Class 10
When you prune the tip of a plant, side branches grow out. The tip normally suppresses lateral growth — this suppression is called apical dominance.